The present invention relates to an RFID tag and manufacturing method thereof, and more particularly an RFID tag that comprises a patch antenna (flat antenna) as the tag antenna, and which is capable of demonstrating the required characteristics even when applied to an object such as a electrically conductive body, electrically non-conductive body or a body including liquid, and to the manufacturing method thereof.
Conventionally, in the distribution industry, transport industry and the like, a method of printing or sticking barcodes on a product itself or on the product packaging, and then reading that barcode with a barcode reader has been widely used as a method for managing individual product information. However, in that barcode processing method, when reading the barcode, the barcode reader must come in contact with the barcode, which makes the work of reading troublesome. Moreover, in the conventional barcode processing method, there is a problem in that it is not possible to add new information or update the information of the barcode itself.
Therefore, recently, a method of attaching an RFID (Radio Frequency Identification) tag to products in the place of a barcode, and reading that product information without contact using radio waves (electromagnetic coupling) is needed and is in the progress of being put into practical use. An RFID tag is a tag that has an IC card function to which a radio communication function for information has been added, and has a non-volatile memory that is capable of storing information, but does not have a battery (power source). Therefore, when reading information from the RFID tag memory without contact, a tag reader is constructed so that it supplies power to the RFID tag using electromagnetic waves, and reads information from the tag memory. With this kind of RFID tag, it is possible to greatly improve workability, and by using a technology such as a verification function or encoding between the reader and RFID tag, it is possible to ensure good security.
FIG. 38 is a drawing explaining an RFID tag, where a reader 1 transmits a radio signal (electromagnetic wave) of modulated data from an antenna 2 to an RFID tag 3. The antenna 3a of the RFID tag 3 inputs the received signal to a rectifier circuit 3b and a modulation and demodulation circuit 3c. The rectifier circuit 3b converts the radio signal to DC voltage, and supplies this DC voltage to the modulation and demodulation circuit 3c and a logic circuit 3d, and that voltage functions as a power supply. The modulation and demodulation circuit 3c demodulates control data that was sent from the reader 1, and inputs the result to the logic circuit 3d. The logic circuit 3d performs logic processing according to the control data (command), for example, reads information that is stored in an internal memory and inputs that information to the modulation and demodulation circuit 3c. The modulation and demodulation circuit 3c uses the information that was input from the logic circuit 3d to modulate a carrier signal, and transmits that signal as a radio signal from the antenna 3a to the reader 1.
Various types of RFID tags have been proposed. One of those is an RFID tag that is constructed by mounting an antenna pattern for radio communication and an IC chip (LSI) on a dielectric base sheet made of plastic, paper or the like. When this kind of RFID tag is attached to an electrically non-conductive object, the desired performance, such as communication distance and the like, is obtained. However, when this kind of RFID tag is attached to metal such as steel, the metal obstructs the radio waves used for communication with the RFID tag, and problems occur such as a decrease in the communication distance.
FIG. 39 is a drawing explaining the occurrence of this kind of problem, where (A) of FIG. 39 shows the case in which an RFID tag having a half-wave length dipole antenna pattern is attached to an electrically non-conductive object (not shown in the figure), and the power (open-circuit V) necessary for the IC chip is generated in the dipole antenna DP by the radio waves emitted from the reader/writer antenna. Also, current I flows in the dipole antenna making it possible to transmit an electromagnetic signal to the reader/writer antenna.
However, when an RFID tag having a dipole antenna pattern is attached to a metal object, the tangential component of the electric field on the metal surface becomes ‘0’ from the boundary condition, and the surrounding electric field becomes ‘0’. Therefore, it is not possible to supply the necessary power to the IC chip of the RFID tag. Also, it is not possible to transmit (scatter) an electromagnetic wave to the reader/writer antenna from the tag antenna. In other words, as shown in (B) of FIG. 39, in the case of an RFID tag having a dipole antenna pattern DP that is attached to a metal object MTL, when current I flows in the dipole antenna DP, an image IMG, in which current flows in the opposite direction, is generated in the metal object MTL according to the mapping principle. This image cancels out the electric field that is generated by the dipole antenna current I, and thus it is not possible to supply the necessary power to the IC chip of the RFID tag, and it becomes impossible to transmit an electromagnetic wave to the reader/writer antenna from the tag antenna. Due to the aforementioned problems, an RFID tag having a tag antenna capable of transmitting and receiving electromagnetic waves without degradation of the antenna gain even when attached to a metal surface is desired.
As shown in (C) of FIG. 39, reducing the image effect by increasing the distance D from the surface of the metal object MTL to the dipole pattern DP is feasible, however, there is a problem in that the thickness of the RFID tag increases. Also, an RFID system in the UHF band has the advantage of having a long communication distance compared with other frequency bands, however, the length of a dipole type tag antenna for the UHF band normally must be a half wave length (approximately 16 cm). This length can be ensured and made more compact by attaching and bending the tag antenna around a dielectric body, however, the bandwidth becomes narrow. Taking the aforementioned problem into consideration, desired is an RFID tag that is small and compact and that has an antenna being capable of large bandwidth without degradation of the antenna gain even when the RFID tag is made small and compact.
Also, in order to efficiently supply the receiving power of the tag antenna to the LSI chip, the impedances of the tag antenna and the LSI chip must be matched (impedance matching). In order to accomplish this, an impedance conversion circuit is necessary, however, that would increase the manufacturing cost of the RFID tag. Therefore, it is necessary to perform impedance matching of the tag LSI and tag antenna without using an impedance conversion circuit. In other words, desired is an RFID tag that has an antenna for which impedance matching with the LSI chip is possible without having to use an impedance conversion circuit.
Conventional RFID tags having a dipole antenna have a problem in that the communication distance becomes poor when the RFID tag is attached to metal as described above. Therefore, some tag antennas have been developed that are compatible with metal even in the UHF band (refer to JP2002-298106A), however all of these are large having a thickness of 4 mm or more and length of 10 cm or more.
Therefore, the inventors of the present invention proposed an RFID tag having a small antenna that is capable of transmitting and receiving electromagnetic waves even when attached to a metal surface (refer to JP2006-53833A). As shown in FIG. 40, this proposed RFID tag 10 comprises: a rectangular shaped dielectric member 11; a transmission/reception antenna pattern 12 that forms a loop antenna around the surface of the dielectric member 11; and an IC chip 15 that is electrically connected to the antenna pattern 12 by way of a chip-mounting pad 13. With the RFID tag having this kind of loop antenna construction, transmission and reception of electromagnetic waves is possible, it is possible to lengthen the communication length even though the RFID tag is thin and is attached to a metal, the gain is nearly constant over a wide band, and furthermore, impedance matching is possible even without an impedance conversion circuit. However, manufacturing an RFID tag having loop antenna construction requires complicated processes such as side surface plating, or processing for wrapping an insulating film around the dielectric member, so there are problems in that the manufacturing cost increases, or high precision is required for positioning the wrapping.
Therefore, recently, use of a patch antenna as an RFID antenna has been proposed. With an RFID tag having this patch antenna construction, there is no need for special work such as side-surface plating or wrapping as in the case of a RFID tag having loop antenna construction.
However, in order to use a patch antenna as an RFID tag antenna, impedance matching with the LSI chip of the RFID tag must be performed. Normally, supplying power to a patch antenna can be done such that power is supplied to the patch antenna from a position that is matched to a 50Ω power supply line, however the impedance of the LSI chip becomes a different value greater than 50Ω, so an impedance conversion circuit is necessary. Also, with a conventional patch antenna it is necessary to make holes in the patch antenna in order to supply power, so there is a problem in that processing cost increases.
An RFID tag patch antenna has been proposed that does not need an impedance conversion circuit, and does not require making holes in the antenna in order to supply power (refer to U.S. Pat. No. 6,215,401). This proposed method is a method that supplies power to the patch antenna in a state in which the tag LSI is DC-connected to this patch antenna, and performs impedance matching by regulating the width and length of the line used for connection, however, it has a problem in that it is easy for construction of the power supply unit to become complicated. Also, in the case of using a board having a low frequency and low dielectric constant, the percentage of space occupied by the impedance matching circuit pattern and quarter wavelength converter with respect to the overall antenna becomes large.